Miniaturization of semiconductor devices, such as field effect transistors (FETs), has been the main driving force behind the incredible progress in the area of computation over the last half decade. Use of smaller FETs advantageously results in a higher packing density, as well as improved device performance. To further sustain this trend, novel nanostructures are explored as possible successors of the current state-of-art silicon devices. One type of nanostructure considered as particularly promising for forming nano-sized FETs is a semiconductor nanowire grown by homo-epitaxy (such as a germanium or silicon nanowire).
However, building devices from semiconductor nanowires requires reinventing the conventional complementary metal-oxide-semiconductor (CMOS) process flow and redesigning desired doping profiles and implementation details, so that such semiconductor nanowires may become the central portions of new high performance nano-sized FETs.
The conventional nano-sized FET contains a rather simple (i.e., non-ideal) semiconductor nanowire structure. Specifically, a top or bottom gate is used to control the electrostatics inside a channel region of the FET, which is located in the uniformly doped or undoped semiconductor nanowire, and source/drain metal contacts for the FET are attached to the two ends of the semiconductor nanowire. In such a FET configuration, the gate not only controls the electrostatics inside the channel region, but also impacts the injection properties at the metal/nanowire interface. This unavoidably results in a transmission probability that is substantially smaller than unity through this interface when an undoped nanowire is used, or in a very substantial shift of the threshold voltage when a uniformly doped nanowire is used for improving the contact quality.
The desired approach would involve a more complicated doping profile with a highly doped region located close to the metal contacts and a lowly doped or undoped region located close to the gate, but away from the contacts.
Currently, there are two standard approaches for realizing such a complicated doping profile in a nanowire—both suffering from substantial drawbacks. One approach is to grow the semiconductor nanowires in situ with a desired doping profile along the length of the nanowire. This approach, however, is incompatible with the requirement for precisely aligning the FET components, such as the gate and the source/drain metal contacts, at a later stage to this doping profile. Consequently, the parasitic capacitance contributions cannot be reduced as desirable, which ultimately limits improvement in the device performance. The second approach is to implant dopants into an existing semiconductor nanowire, using conventional dopant implantation techniques. It is highly desirable to use nanowires in a three-dimensional integrated approach, but the implanted dopant cannot form a uniform doping profile in various depths of the nanowire relative to the implantation direction. In addition, it is highly questionable whether it is possible to use an ion implantation approach for any nanostructure, since the ions would have to be “stopped” very precisely within the nanostructure.
There is therefore a continuing need for improved nanostructures with desired doping profiles that are suitable for forming FET devices.